In many areas, a reactable gas is used as a processing agent to treat a liquid. Examples of this include water treatment to remove waste or to create potable water and chemical oxidation (i.e. bleaching) processes.
In such processes, it is important to ensure that treatment of the liquid with the reactable gas continues for a sufficient period that the desired treatment result is achieved. In water treatment applications, commonly used reactable gases include ozone and hydrogen peroxide. Ozone is used in many water treatment applications to remove impurities. It is important to ensure that ozonation of the water continues until the level of impurities has fallen to an acceptable level. One method of doing this is to fix the volume of water and then ozonate the water for a period that is known to be sufficient to reduce the impurity level, regardless of the initial concentration of impurities in the water. However, this method may waste ozone (if the initial level of impurities was relatively low) as well as requiring a fixed, and possibly lengthy, time for each ozonation process. It is preferable to use a system that monitors the impurity level and stops the ozonation process when the acceptable impurity level is achieved.
Accordingly, various different sensors have been developed to measure the level of ozone in water. Some of these sensors operate by passing ultraviolet light through a fluid stream and measuring the ultraviolet light received on a detector. Another type of gas detector is disclosed in U.S. Pat. No. 5,167,927 to Karlson. Karlson discloses a monitor which measures the heat energy released when a gas, e.g. ozone, is catalytically converted into a different compound, e.g. oxygen. A third type of sensor is disclosed in U.S. Pat. No. 5,427,693 to Mausgrover et al. Mausgrover incorporates a meter to measure the oxidation-reduction potential (ORP) of the water being cleaned. The ORP is then equated to an ozone concentration in the water. A fourth type of ozone sensor is disclosed in U.S. Pat. No. 5,683,576 to Olsen. In the system described by Olsen, an ozone containing gas is passed through contaminated water until the concentration of ozone in solution in the water reaches a pre-determined level. Ozonation then continues for a pre-determined period. The objective of this system is to ensure that a specified volume of water will be treated with a specified concentration of ozone for a specified period of time.
Although these systems may provide a reliable measure of the concentration of ozone in water, none of them provides an accurate measure of the degree to which impurities have been removed from the water. Continuing ozonation after the desired ozone concentration is reached for a pre-determined period ensures only that a minimum amount of ozone passes through the water over the entire treatment period. Olsen assumes that once the concentration of ozone reaches the predetermined level, it does not subsequently fall. Further, Olsen assumes that simply allowing a selected concentration of ozone to remain in the water for a selected time ensures that the water is suitable for use. However, this will not necessarily be true, especially in the case of highly contaminated water. For example, lake or well water will normally require more treatment than treated water from a municipal supply.